Wood membrane hygrometer



July 6, 1954 J. w. cAsE 2,682,858

WOOD MEMBRANE HYGROMETER 2 Sheets-Sheet 1 Filed Dec. 13 1951 INVENTOR JAMES W. 0485 QEO W ATTORNEYS July 6, 1954 J. w. CASE 2,632,358

v WOOD MEMBRANE HYGROMETER Filed Dec. 15, 1951 2 Sheets-Sheet '2 -fl--. nu-J.

INVENTOR v JAMS n: was":

W W f5 ATTORNEZS Patented July 6, 1954 UNITED STATES ATENT cr nes W001) MEMBRANE HYGROMETER James W.--Case, Fairfax, Va.

Application December 13, 1951, Serial No. 261,570

(Granted under Ellitle 35, U. S. Code (1952),

see. 266) This invention relates to "improvements in hygrometers, and more particularly relates to devices :for determining the relative :amount of moisture in the atmosphere by means .of thin wood lmem'brane sensitive elements.

The purpose LQf this invention is to provide an inexpensive, accurate, rugged instrument which can be relied upon for visualindication of the relative humidity inside of packages, containers, shipping cases, ig'loo's, buildings, ;etc., in which perishable material is shipped or stored in a :dehumidified atmosphere .as .a means of preservation. a

The technique of preserving perishable equipmentbymeansof dehumidified air "was developed during and succeeding WorldWar II, when itwas found by numerous investigators that rusting and corrosion of.metals isgreatlyireduced'in-an atmosphere below 50% relative humidity; that positive protection against rust :and corrosion is obtained inairbelow 30% relative humidity and that most forms of mildew are prevented from growing below 70% relative humidity. These discoveries led to the development of standardized procedures for dryair preservation which are being widely practiced by the .military services .and commercial contractors.

Essentially, the procedure consists of enclosing the material or equipment being preserved in a moisture .vapor barrier withsilica-gel, which takes the moisture outof the enclosed airspace, either by static adsorption or by stripping the moisture from the air which is .circulated through the silicagelibed by mechanical means.

Since :the .moistureyapor barriers of such enclosures permitacontinualrsmall leakage of moisture from the outer atmosphere into the=enclosures, some means :of occasionally checking the moisture qualityoi the enclosed atmosphere is a necessary adjunct to this preservation process, thereby to inform maintenance and inspection personnel whetheror not'the atmosphere of the enclosure is below the safe upper critical limit of relative humidity andassure adequate protection of the material under preservation.

Presently available means for the indication of .the relative humidityin dehumidified shipping packages and containers are inadequate because they are either too costly and cumbersome, or because they are not accurate within the critical values of moisture which must not be exceeded.

One conventional meansutilizescobalt-chloride impregnatedsilica-gel to indicate the moisture condition of the container atmosphere by certain characteristic colors, depending upon the atmos- '2 pheric relative humidity 'at which the silica-gel is in moistureequilibrium. .This indicating means is not only limited $0 a. qualitative indication of the-mcisture-inthe air, but is also subj ectto-ialse indication, due to its gradual accumulation of moisture with eachzdaily'peak of relative humidity (caused by atmospheric temperature change in the container). This false indication results in unnecessary expense in the opening of the enclosure to inspect the contents, renewt-hesilica-gel chargeetc.

Another available .means employs mechanical hygrometers having sensitive elements of human hair, animal membrane, paper-metal laminate, etc. While such instruments give considerably more accurate indication of the relative humidity than the silica-gel indicators above described, they have not been generally adopted .for use shipping .andstorage packages, because they are too expensive in comparison with the value of material in the: individual. containers or packages; lack the desirable .compactness for adaptation; aresubj-ect to accumulative zscale errors due -.to aging .efiectson their sensitive elements and are in general too frail to withstand the shock and vibration to which packages and containers are subjected during shipment. V

I The present invention -.contem;plates the use of the hygroscopicity; of woodand moreparticularly, the distortional characteristics of thin wood membranes in imoisture equilibrium with the atmosphere :as moisture sensitive-elements in lowcost, compact, reliable hygrometers, humidity indicating devices, controllers,-etc.

The constants-of the characteristic. deformation of various species of wood withchanges .of moisture content have been well established during the practice of .kiln drying'of lumber by the lumber industry andiinresearchicarried out by the Forest Products Laboratory 'of the U. S. Department of Agriculture. Toouote the latter authority:

Wood shrinks most in the direction of the annual growth ring-s tang-enti ally) about onehalf vas-muchlas-this across these .rings (radially) and very little, sisal-rule, along the grain (longitudinally) The shrinkagethat occurs when the moisture content isreduced below the fibersaturation point is rbasically dependent upon the amount of moisture removedfrom the cell Walls. For :practical purposes, swelling may be considered the reverse of rshrinkage. For purposes of calculating-shrinkagaor swelling, the moisture content-shrinkage relationship may be consideredaadirect one.

For reasons which are explained later, the

selection of the radial shrinkage and swelling of wood are characteristics which can be depended upon for accurate indication of the amount of moisture in the atmosphere by means of thin wood membranes. Further, there have been selected species of wood which are durable; which are easily worked and which have sufficient moisture-distortion gradients for actuation of indicating means in mechanical hygrometers. For example, the unit radial distortion of three typical species of wood, over the full range of relative humidity are calculated from Forest Products Laboratory data and are indicated in the following table.

Unit radial distortion of wood in moisture equilibrz'um at various relative humidities and 70 F. (DB).

It has been found that the moisture distortion characteristics of wood, above cited, are not useful for the moving force in a mechanical hygrometer in a thick section cut from the tree, as in a piece of lumber, but the sensitive element must be made in a very thin membrane which can rapidly respond to changes of the atmospheric moisture. According to the literature, wood was one of the first materials utilized in the construction of hygrometer sensitive elements. However, all types of wood sensitive elements employed by previous inventors are of relatively thick sections, in which an appreciable time is required for equal wetting of the mass and swelling of the internal fibers. This circumstance causes objectionable time lag between changes of atmospheric moisture and the desired dimensional change of the sensitive element. The inherent time lag and sluggish nature of wood sensitive elements employed by previous inventors has generally prevented the use of this type of element in commercial hygrometers which normally employ human hair, animal membrane, paper-metal laminate, etc. as sensitive elements. It has been found, further, that all such organic fibers have an inherent time lag of at least 40 minutes between changes of relative humidity and that, for comparable performance, a wood fiber must be of approximately the same thin section, or at least, as thin as practicable within the limits of the mechanical fac tors incident to the service of the instrument.

There has therefore been produced a wood hygrometer sensitive element of comparable thinness, accuracy and sensitivity to other organic fibers which are employed in hygrometers.

From those statements it will be understood that one of the objects of the invention is to provide an improved moisture indicating instrument for the guidance of shippers and inspection personnel who are charged with the safe transit or storage of valuable equipment which is subject to rusting, corrosion and mildew and which is preserved by means of moisture vapor barriers and dry air.

Another object of the invention is to provide a moisture indicating device which is contrived to give warning, by means of a visual mechanical signal, when the critical value of relative humidity in dehumidified shipping packages or containers is reached.

Another object of the invention is to provide moisture sensitive elements of thin wood membranes for hygrometers and hygrosignals.

Another object of the invention is to provide moisture sensitive elements for hygrometers and hygrosignals which are resistant to the eiiects of solvent vapors, such as methyl-ethyl-keytone, ethyl alcohol, and petroleum solvent which are commonly used in moisture barriers and in metal preservatives.

Another object of the invention is to provide moisture sensitive elements for hygrometers which will have uniform moisture-distortion characteristics when produced in quantity, so that the hygrometers in which they are used may have duplicate scales and will assure acceptable accuracy over the full range of relative humidity.

Another object of the invention is to provide simplified mechanical magnification means for hygrometer sensitive elements to eliminate accumulative instrument errors due to strain on the elements and shock or vibration during service.

Another object of the invention is to provide a tripping and resetting means for a mechanical signal which are operated at preset values of relative humidity by moisture sensitive elements.

Another object of the invention is to provide moisture sensitive elements which are responsive with a minimum of lag to the fluctuating moisture vapor conditions in the atmosphere.

Another object of the invention is to utilize the steps of a method of producing the critical membraneous-wood tube which is the sensitive element of the hygrometer.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a perspective view illustrating the first mechanical step of the method of producing the sensitive element;

Fig. 1A is a diagrammatic perspective view showing the method of cutting a block from a log preparatory to obtaining a wood membrane which is expansive and contractive radially in the tree;

Fig. 2 is a plan View of the membraneous-wood shaving, stretched out preparatory to cutting;

Fig. 3 is a perspective View of a tube wherein the membraneous wood sections are joined for rectilinear expansion and contraction;

Fig. i is a diagram illustrating a development of a spiral winding of the wood membrane for rotational expansion and contraction;

Fig. 5 is a side elevation, partly in section, of a modification of the hygrometer of the immediately preceding type employing a trippable signal flag;

Fig. 6 is an enlarged section of the trip end of the latter hygrometer;

Fig. 'l is an elevation of the hygrometer in Fig. 7, showing the trigger and flag in the unset position;

agoeagsss Fig; S is-a detail perspective view, particularly showing-theundercut of the-trigger;-

Fig-.- 9-15 a partially elevational and sectional view of a modification of the hygrometer in- Fig. 6:

Fig: 10"- isan elevation of one end of thehy grometer'in Fig. 9;

Fig. 11 is anelevati'onof theotherend of the hygrometer in Fig. 9';

The method of making the sensitive element is described first, thenits applications In Fig; 1 I designates a block of close-grained, well seasoned'soft wood. Species that areregardedas suitable for theinstant purposein'clude' White Pine (Pinus strobus), Yellow Poplar (Lir-io'den dram tulipifera) and Red" Cedar (Jflniperus-virginianay. The-block Ill; Fig. 1 is cutfromalog 1A according-to the common commercial practice of quartersawing. to bring outthe radial grain. of the wood. Asshown in Fig; IA, the-block I0 is cut from tlietimber'so that the faces Illa; IOU of the block are purposely inparalleL orientation with the radiall plane aa bb. Now when a shaving is cut from theface I 017, Fig. 1, in the direction of thegrain and across the ring growth laminations, there is producedathin membrane I i, Fig. 2; which utilizes only: the radially distortional constants of the wood along the line AB, Fig. l. Thismann'er of cutting the block insures a uniformity of distortion in: the active element of the hy-'-- grometer which could not be obtained: in anyother known way.. It is this uniformity of dis tor-tion: which .is responsible for' the" manner of response. of the hygrometer element to ambientmoisturezas set. out herein. Thedesired specimen having been selected, a shaving I I: iscut from one side inthe manner described above.

Asharpcarpenters plane is used for this purpose, or any equivalent instrument-that will d care has to be taken in the next step:of smo0th-- ing the; shaving. out: as in.Fig. 2 andlcutting from it, sections ll. of: uniform. length. The cuts. are made along the lines. I 3;.

These. sections: ii. are assembled on a .mandrel.

(notwshown), and united at. the joints IL with an alkyd:resin',.cellulose nitrate, or phenolic cement,

available commercially. The longitudinal. joints I are.=desirably staggered around the mandrel. The result: is the cylinder. I61 in: which the; joints. furnish; reinforcement to the :otherwise frail i sections. This cylinder is the: sensitive. element; Now; since the sections l2 are assembled: endv on-endwiththeir axesicorresponding, in the tree, to.-the, radial plane aabbhliig. lAand the line. A-.-B.Fig. 1,v the cylinder l6, Fig. 3, forms. a moisture-sensitive elementwhich. is responsive only to the radially expansive. constant of. the. tree wood. When the humidity is. on the. increase the cylinder willl expand lengthwise and as the moisture content dimini'shesthe cylinder will contract. Tests-'to date show that a maximum dimensional change of" anelement as shown in Fig. 3"when made of- White Pine=isonthe order of- .002 inch per-inch length.

Thee modification i'n Fig; 5 hasxtl'ie trippable' 1 of thettube 42.

free. end is anchoredLto a stop pin 41'.

flag: 28- (Figs. 6 and '7) forthe signal orindicat i'ng element. Thisfiagis a. disk, one half of which; is painted red, the other half green. or" some optionally: contrasting colors. The disk is fixed on a spindle 2.9 which becomes; a push pint The push pi'n isslidably carried by the cover-30. of the screw cap-3i which is cut away as at" 32" (Fig. 7) to providea window and abearingfor. the push pin;

A pad: 311 at theinner end of the push pin con:- fi'ontsla: plug- 35 in: the near end of; the. mem:- branoussensitive element It. The oppositeend; of the element hasaplug. 35a resting against a boss. in which forms a mounting inthe-bottom.

The outer end of. the pin 29. can-'- ries' atrigger 3.5. -the free end of whichis. angled; downwardly: at 3.75 to engage the edge otthecuts away 3-2i to hold the trigger in set'position. The anglediend. is desirably undercut at.38: to insure; a sharp edge for.v contact withthe equallyisharp' edge. of the. cut-away 32. A counterclockwise. turn of thetrigger 36. from the unset position in. Fig. 7 registers .the sharp edge of the end 81 with. the cut-away: whereupon the trigger is caught.

- andheld.

Thespring 40: functions inthe setting of the; trigger and in its return to the unset position; (Fig; '7). It iscoiled around the: spindle 29; one end. is fastened" to. the cover 30, the other and; In practice the: coil of: the spring. exercises a degree of axial. expansion: and a consequent endwise' pressure. This both keeps the pad 34 in contact with the plug 3-5} only at times, however, andicauses' the.

' knife-edge of; 3] to. spring out of engagement:

with'the cut-away 321 The pin is fixed in the; fiag.28;.stands erect: therefrom and strikesthe. edge of the cut-aways! when the trigger isin the: unset. position.

The: outer; perforated metal tube. 42 which. fnrmszthe-casing for most of the hygrometer in Fig; 5",- has athread course 43 (it is much" like a flashlight casing). onto which the cap 3! is. screwed; A1. scriber or similar mark it on the V screw cap is readable against the graduations t on the; tube and thus a setting of. the hygrometerr'fora. chosen humidity condition is made. Assume, for example, that an am bient. humidity is determined upon for the signaling'of-the flag 281 The cap. is turned until. the hygrometer showsthat reading.

It: depends on; the moisture content of the sensitive: element l6. whether or not the pad 3'4: willitouch; Onrthe. assumption that the ambient" humidity was less than 80% the pad will stop short of: the plug. 35, as shown. Now, as the sensitive elementxtakes on moisture, it expands; axially: and at: the designated time pushes the. knife edges! offiof the edge of the cut-away 32'. Theespring; 401thentakes over, swinging the flag from its set (green) position to the danger (red) position Fig. '7.

If the moisture content of the sensitive element; happened to be more than corresponded to an ambient. humidity of 80% it would not have been possible to adjust thecap 3i to the stated setting. The plug 35 would be supporting the spindle 29 and trigger 36 at an elevation so that the knife-edge 37 could not catch the edgeof the cut-away even if swung into registration with'it. If the ambient humidity were less than the chosen 80 allof the foregoing collateralmechanical conditions would be reversed. Consequently the trigger would set" and. stay set until; an: 80% humidity: condition was reached.

7. Figs. 9, l and 11 illustrate a modification of the structure in Fig. 5. Each has a trigger-held and released flag and in that respect they are, broadly, alike. The distinction is that in Fig. both the flag and trigger turn in the same direction on the axis of the hygrometer in pursuance of the release of the trigger upon the axial elongation of the sensitive element by the accretion of moisture, whereas in Fig. 9 both the flag and trigger swing laterally and in opposite directions when the trigger is released in response to said elongation. The perforated metal tube 4'! now has a closure 48 for one end, made either thick enough or formed otherwise to support a thread course for the screw stem of a knurled, humidity-setting nut to which then becomes a mount for one end of the sensitive element.

To the latter end the nut 49 (Fig. has a scriber or equivalent mark 50 which is registrable with the graduations 5|, numbered in percentages of humidity, as said nut is turned. This turning adjusts the support of the sensitive element ii: on the inside of the tube. As in Fig. 5, the ends of the element are plugged, the plug 53 in the end opposite to closure 48 being in more or less close but operative proximity to the pad 54 of the trigger 55.

Said trigger is pivoted in a slot 56 extending through to the open end of the tube and has a sharp-edged angled end 51 and a spring 86, to

hold the angled end 5'! in engagement in the set position. The angled end 51 is engageable with the equally sharp edge of the flag 59 which, like theflag 28 in Figs. 6 and '7 is painted red and green; red on the inner surface, green on the outer surface. When the hygrometer is set as in Fig. 9 the flag shows green. When the flag i tripped as in Fig. 11 it shows red.

A spring GB is so assembled on the pivot spindle ii of the flag as to urge the flag up to the signaling position when released by the trigger. In actual practice, the hygrometer in Fig. 9 is capable of being made in a form much on the order of a pencil, that is to say, long and slender. This facility adapts it to purposes for which the form in Fig. 5 may prove not as well suited. But either of these two types is Well adapted to embodiment in packages of equipment which is to be guarded against an excess of humidity. They are intended to be so mounted that the signal flag is visible through a glazed window in the package.

Fig. 4 illustrates the first and only modification oi the sensitive element itself. Here, instead of being made in sectional form the shaving in Fig. 1 is left intact and is formed into a unitary wood shaving i8, spirally wound into a tubular element and made to remain so by spots of cement at intervals. The purpose is accomplished, for example, by winding the shaving spirally on a mandrel and lightly cementing the edges at points about apart with cellulose nitrate, alkyd resin or phenolic cement.

The object of this is to produce a non-rigid tube which will permit respective rotation between the two opposite ends, due to spiral distortion upon the accretion and loss of moisture.

It has been found that such a tube 6" long will rotate approximately 30 between opposite ends from ambient conditions of between 50% and 100 relative humidity.

In further explanation of Fig. 4, the rotative effort caused by the expansion (or contraction) of the wood membrane will be understood from the development of the spirally Wound element. In accordance with the transverse grain expansion set out before, the maximum dimensional change of the developed sheet is diagonally between points 2) and d. This represents adistortion of the rectangular sheet into a parallelogram as shown at a'bcd. With points 0d fixed, as they will be by the cementing down of the tube, the movement of points ab to a'b corresponds to clockwise motion at the free end of the tube, in other words to an extent where the element is in equilibrium with the particular moisture content of the ambient air,

The operation of the various forms of the hygrometer is generalized thus: The accretion or loss of moisture by the wood-membrane sensitive element I6 causes a dimensional change therein which is utilized to shift the indicating needle. This occurs either by axial elongation or contraction or by the twisting of the tubular body. The humidity indicator (flag or needle) is moved by spring-trigger action (Figs. 5 and 9).

The invention described herein may be manufactured and used by and for the Government of the United States of America for governmental purposes without the payment or any royalties thereon or therefor.

What is claimed is:

1. In a hygrometer, a moisture sensitive element consisting of a tubular membrane, a casing in which said element is contained, mounting means in the casing for one end of the element the other end of the element being free to move, a spring-loaded signal sWingably carried by the casing, and a trigger mounted between the signal and the casing, said trigger holding the signal in a safety-indicating position and having a portion confronting the free end of said element.

2. In a hygrometer, a moisture sensitive element consisting of a tubular membrane, a casing in which said element is contained, one end of the element being relatively stationary the other end or" the element being free to move, a spring-loaded signal having a spindle turnably and slidably carried by the casing and having a portion confronting the free end of said element, and a trigger mounted on the spindle, having an end engageable with a portion of the casing upon sliding and turning the spindle against the spring loading to establish the signal in a safety-indicating position.

3. In a hygrometer, a moisture sensitive element consisting of a tubular membrane having an active end and an inactive end and being plugged at its active end, a casing in which said element is contained, the inactive end being relatively stationary the active end being free to move, a spring-loaded signal having a spindle, one end of the spindle having a pad confronting the plugged end, the other end of the spindle having a trigger with an angled end directed toward the signal, and a cap having a cover in which the spindle is turnably and slidably carried, being cut away to provide a window through which the signal is visible and an edge at which the angled end is engageable upon sliding and turning the spindle against the spring loading to establish the signal in a safety-indicating position, said cap being screwed onto the casing for adjustment to vary the initial setting of the pad in respect to the plugged end.

4. A moisture sensitive expansible and contractible element for a hygrometer or the like comprising a thin walled open ended tube formed of a single ply of seasoned wood shaving and .1 having uniformly parallel grain fibers encircling the tube.

5. A moisture sensitive expansible and contractible element for a hygrometer or the like comprising a thin walled open ended tube formed of a single ply of seasoned wood shaving and having uniformly parallel grain fibers continuously encircling the tube and having adhesively joined lapping end portions longitudinally reinforcing the tube.

6. In a hygrometer, a moisture sensitive element consisting of a tubular membrane, a casing in which said element is contained, mounting means in the casing for one end of the element the other end of the element being plugged and free to move, a spring-loaded signal swingably carried by the casing, and a trigger-pivoted on the casing, said trigger having a pad confronting the plugged end of the element and having an angled end holding the signal in a safety-indieating position. 7

'7. In a hygrometena moisture sensitive element consisting of a tubular membrane, a casing in which said element is contained, said element having an active end which is plugged and free to move, a spring-loaded signal swingably carried by the casing, a trigger pivoted on the casing, said trigger having a pad confronting the active end of the element and having an angled end holding the signal in a safety-indicating position, and a means against which the other and also plugged end of the element is mounted, said means being adjustable to shift the element thus to vary the initial setting of the trigger pad in respect to the plug in the active end of said element.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,523,322 Walton Jan. 13, 1925 1,675,302 Roemer June 26, 1928 1,958,812 Bristol May 15, 1934 1,965,705 Lamlein et a1. July 10, 1934 1,970,745 Goss Aug. 21, 1934 2,031,408 Eggleston Feb. 18, 1936 2,369,215 Crise Feb. 13, 1945 

